Alkaline earth metal nitrides such as Ca3N2, Sr3N2, Ba3N2 or the like are compounds which are used as raw materials of aluminum nitride for semiconductor devices, ceramic particles for metal sliding members, battery electrode constituent materials, conductive fine particles, or the like. Patent Document 1 discloses a method of producing an alkaline earth metal nitride by thermally decomposing a corresponding alkaline earth metal amide. In addition, Patent Document 2 discloses a method of producing a high purity metal nitride by reacting ammonia with an alkaline earth metal to make it liquid-phase, and then thermally decomposing the obtained metal amide compound.
Patent Document 3 discloses a method for producing a metal amide compound by reacting a metal hydride or a metal hydride in which a simple substance metal or an alloy is further added with liquid ammonia. As another method of producing metal amide such as LiNH2, Ca(NH2)2 or the like by enclosing a metal such as Li or Ca or a compound thereof in a reaction vessel, and after cooling, introducing ammonia having a volume ratio of 10 times or more with respect to metal and liquefying the ammonia, and then reacting them while stirring (Patent Document 4).
Calcium nitride which is a typical alkaline earth metal nitride is known as α-Ca3N2, β-Ca3N2, γ-Ca3N2, Ca11N8, Ca2N, or the like. Ca2NH, CaNH, Ca(NH2)2 or the like which are hydrides of calcium nitrides (hereinafter also referred to as “Ca—N—H based compounds”) are also known.
It is known that Ca2N is a very chemically unstable substance. For example, it is readily oxidized. It has been reported that Ca2N can stably exist at 1000° C. or less in Ar, or between 250° C. and 800° C. in nitrogen (Non-Patent Document 1).
On the other hand, the present inventors have found that a nitride represented by AE2N (AE represents at least one element selected from Ca, Sr, and Ba) is a “two-dimensional electride compound” having high conductivity (see Patent Document 5). The two-dimensional electride compound AE2N is a layered compound in which electron (e−) is bound as an anion between layers composed of [AE2N]+. That is, it can also be expressed as AE2N+:e− in the ionic formula.
For example, Ca2N, which is a typical two-dimensional electride compound, is obtained by heating Ca3N2 and metal Ca in vacuum. It has been reported that the conduction electron concentration of Ca2N is 1.4×1022/cm3 and has a work function of 2.6 eV (Non-Patent Document 2). Thereafter, an example in which this two-dimensional electride is used as a reducing agent for pinacol coupling has been reported (Non-Patent Document 3).
It has been reported that Ca(NH2)2 acts as a base catalyst and exhibits catalytic activity against an isomerization reaction of olefins such as 2-methyl-1-butene (Non-Patent Document 4). Another example is a catalyst in which an amide compound of Na, K, Eu, Yb is supported on an oxide support such as Al2O3. It exhibits catalytic activity for an olefin isomerization reaction such as 2-methyl-1-butene has been reported (Non-Patent Document 5). It has been reported that each example functions as a base catalyst.
For ammonia synthesis, a method using a catalyst containing Fe3O4 and several mass % of Al2O3 and K2O in Fe3O4(Haber-Bosch method) is generally used. In addition, iron-based catalysts and Ru-based catalysts (for example, Ru/MgO, Ru/CaO, Ru—Cs/MgO) have been studied as synthesis methods other than the Harbor-Bosch method (Non-Patent Documents 6 and 7). These catalysts are catalysts in which a transition metal having ammonia synthesizing activity is supported on a support, and are generally referred to as “supported metal catalysts”.
Other supported metal catalysts for ammonia synthesis include transition metals of Group 8 or 9 of the Periodic Table such as Fe, Ru, Os, Co, nitrides of transition metals of Group 8 or 6B of the periodic table, composite nitrides of Co.Mo and the like are used (Patent Documents 6 to 9). Also, an ammonia synthesis catalyst is known in which Al2O3, SiO2, Mg2O or magnesium aluminum spinel is used as a sub-support and Ru is supported on silicon nitride or boron nitride supported thereon (Patent Document 10).
Then, the present inventors have found that a transition metal supported on the two-dimensional electride becomes an ammonia synthesis catalyst having high activity. Specifically, a supported metal catalyst in which a transition metal such as Ru or Fe was supported on a metal nitride represented by MxNyHz (M is Mg, Ca, Sr, And Ba, x is an integer satisfying 1≤x≤11, y satisfies 1≤y≤8 and z satisfies 0≤z≤4.) or its hydride is a catalyst for ammonia synthesis (Patent Document 11). However, there is no report regarding a composite in which a metal amide compound and a metal are supported on a support, and a supported metal catalyst of the same.